Wavelength-Division-Multiplexing (WDM) is an attractive option for providing increased capacity in light wave transmission systems and routing capability within optical networks. For example, high capacity transport systems that carry as many as eight wavelengths per fiber have been developed and are currently being deployed. Presently in these systems, each transmitter includes a laser that is intended to operate at only one of the allowed wavelength channel frequencies. The lasers are engineered to operate within the channel specifications for the life of the system by both tight control of the laser fabrication and its operating environment. To plan for the event of a failure, an inventory of transmitters for each wavelength channel must also be maintained. As undoubtedly the demand for the number of wavelengths in transport and local systems will increase to 32 or more in the not too distant future, new technological capabilities and designs for transmitters and receivers will be required to minimize the complexity and cost of the use of such large numbers of wavelengths.
Several approaches to address the channel stabilization and inventory issues are being considered. One solution to the inventory problem is to use a tunable laser that can be adjusted to the desired channel within each transmitter. However, tunable lasers are more complicated than fixed frequency lasers, will naturally drift over a wider range, and will require stabilization of the operating wavelength for each of a large number of channels. A very attractive solution to the inventory and stabilization dilemma is to use a wavelength selectable laser. In this case, each photonic source chip includes several fixed frequency lasers, which the desired channel may be selected from. Wavelength selectable source chips that can reach 4-6 channels have been reported (see M. G. Young, U. Koren, B. I. Miller, M. Chien, T. L. Koch, D. M. Tennant, K. Fedder, K. Dreyer, and G. Raybon, "Six wavelength laser array with integrated amplifier and modulator," Electron. Lett., vol. 31, no.21, pp. 1835-1836, Oct. 12, 1995), however reaching larger numbers would presumably present significant yield and packaging issues.
Recently, an alternative approach to multi-wavelength sources that can generate many or all of the system wavelengths has been reported. A multi-wavelength source that has been used in many return-to-zero system experiments is the super-continuum laser in which the discrete spectrum of optical pulses from a mode-locked fiber laser is broadened and made continuous by non-linear processes in a dispersion-shifted fiber (DSF)(T. Morioka, "Supercontinuum lightwave optical sources for large capacity transmission," Proc. 21 st. Eur. Conf. On Opt. Commun., Brussels, 1995, paper Th.A.1.2, pp. 821-828). The desired wavelength channels are then selected with an optical filter. These wavelength channels are not CW but pulsed and, therefore, are useful only for return-to-zero systems.
A procedure that generates a spectrum with a precise and stable frequency spacing between the components, or wavelength channels, is the periodic modulation of light. In this case the channel spacing is equal to the repetition rate of the modulation. However, it is difficult to modulate light at the required frequencies of greater than several tens of GHz and of a sufficient magnitude to produce many useful wavelength channels. A reported variation of this approach employed a monolithic mode-locked laser to produce pulses at a repetition rate of 50 GHz and, hence, discrete CW frequency components with a separation of 50 GHz (H. Yasaka, Y. Yoshikuni, K. Sato, H. Ishii, and H. Sanjoh, "Multiwavelength light source with precise frequency spacing using mode-locked semiconductor laser and arrayed waveguide grating filter," in Tech. Dig. Conf. on Opt. Fiber Commun., San Jose, 1996, paper FB2, pp. 299-300).
Another variation of the modulation technique is to couple CW light from a laser into an optical resonator which contains an optical phase modulator to modulate the cavity length (T. Saitoh, M. Kourogi, and M. Ohtsu, "A waveguide-type optical-frequency comb generator," IEEE Photon. Technol. Lett., vol. 7, no. 2, pp. 197-199, February 1995). For efficient operation, both the mode-locked laser and optical resonator techniques require that the modulation frequency be an integer multiple of the free spectral range of the cavity, thereby, placing stringent requirements on the cavity length and modulation frequency.